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 A137815 Year numbers: numbers n such that phi(n) = 2 phi(sigma(n)). 7
 5, 13, 37, 61, 65, 73, 119, 157, 185, 193, 277, 305, 313, 365, 397, 421, 457, 481, 541, 613, 661, 673, 733, 757, 785, 793, 877, 949, 965, 997, 1093, 1153, 1201, 1213, 1237, 1321, 1381, 1385, 1453, 1547, 1565, 1615, 1621, 1657, 1753, 1873, 1933, 1985, 1993 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Following D. Iannucci, n is called a "year number" if phi(n) / phi(sigma(n)) = 2 (thus 365 is a year number, explaining the terminology). D. Iannucci asks: Are there any even year numbers? Are there any odd year numbers that are not squarefree? Remark: If n = q_1 q_2 ... q_k is a product of odd primes such that (q_j + 1)/2 is an odd prime for all j, then n is a year number. Solution: for nonsquarefree year numbers, see A137816. See A137817-A137819 for year numbers with cubes, 4th powers, 5th powers. Eric Landquist found year numbers divisible by 7^2, 7^3 and 7^4, as well as 120781449 = 3^8 * 41 * 449. The existence of even year numbers is still open, but Eric checked all 200-smooth even integers with a single large prime up to 10^8 and found no year numbers among them. See also references in A082897 (perfect totient numbers). REFERENCES R. K. Guy, "Euler's Totient Function", "Solutions of phi(m)=sigma(n)", "Iterations of phi and sigma", "Behavior of phi(sigma(n)) and sigma(phi(n))". =A7 B36-B42 in Unsolved Problems in Number Theory, 3rd ed. New York: Springer-Verlag, pp. 138-151, 2004. Doug Iannucci, in: Gerry Myerson (ed.), 2007 Western Number Theory problems set. LINKS M. F. Hasler, Table of n, a(n) for n=1,...,7499. MATHEMATICA Select[Range[2000], EulerPhi[#]==2EulerPhi[DivisorSigma[1, #]]&]  (* Harvey P. Dale, Mar 18 2011 *) PROG (PARI) for( n=1, 10^7, eulerphi(n)==2*eulerphi(sigma(n)) && print1(n", ")) CROSSREFS Cf. A137816-A137819, A006872 (phi(sigma(n)) = phi(n)), A067704 (phi(sigma(n)) = 2 phi(n)), A082897. Sequence in context: A238460 A342475 A107144 * A089523 A058507 A111057 Adjacent sequences:  A137812 A137813 A137814 * A137816 A137817 A137818 KEYWORD easy,nonn AUTHOR R. K. Guy, R. J. Mathar and M. F. Hasler, Feb 11 2008 STATUS approved

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Last modified May 15 17:50 EDT 2021. Contains 343920 sequences. (Running on oeis4.)